Transgenic mice offer a unique approach to study gene regulation and function during mouse development. Previously, transgenic mice have been generated with various genes that have been properly regulated under the control of tissue-specific promoters/enhancers. Ets is a superfamily of genes and its members (ETS1, ETS2, ERG, ERGB, PU.1, E74A, SAP-1, ELF-1, PEA3) encode transcription factors that activate transcription via binding to purine-rich core sequence GGAA in various promoters/enhancers of cellular and viral genes. In order to study the role of the ets-2 gene during mouse development, a transgenic line containing the mouse ets-2 gene in the germline (ets-2 mice) was generated. The transgene was expressed in testis only and the ets-2 homozygous mouse developed hydrocephalus. These mice had atrophic thymus and an enlarged brain and died within 2-3 weeks of birth. Since, initially, there was only one transgenic line, it is difficult to conclude whether the physiological defects observed were due to the ets-2 trans- gene expression or due to the integration of the transgene into one of the important mouse genes. Therefore, it was set out to generate additional transgenic lines using the same ets-2 fragment, as well as the ets-2 gene linked to another promoter, such as beta-actin. The results obtained from new transgenic lines should allow confirmation of the initial observation, as well as provide new insight into the role of ets-2 during mouse development.